Recent advances in the field of biomechanical energy harvesting have led to the development of devices that are capable of capturing mechanical energy produced by human motion and converting that energy to electricity. Existing devices can be grouped into three categories based on the principle used in the energy conversion: (a) Inertia-based, which use inertia force of a proof mass as the input for power generation; (b) Impact force-based, which use the impact force of a moving mass to drive a generator; and (c) Motion-based, which directly harness limb motion and use it to drive a generator.
An inertia-based system that generated a relatively large amount of power was a suspended-load backpack which captured the up-and-down movement of the carried load during walking to drive a rotary-magnetic generator (Rome, L., et al., “Generating electricity while walking with loads,” Science, 309:1725-1728, 2005). This system produced approximately 7.4 W of electrical power from a 38 Kg load during fast walking and approximately 0.5 W of electrical power at more modest loads and speeds. However, a drawback of this system was the requirement for the user to carry a substantial load to generate a modest amount of power. Furthermore, the system's up-and-down oscillating mass may disrupt the user's gait pattern and walking stability.
Impact-force-based systems have primarily focused on exploiting the heel impact during walking and attempt to capture the energy that is normally dissipated. For example, such systems harvest energy from the compression of the shoe sole as the leading leg accepts the body weight during walking. Designs have been proposed based on a magnetic rotary generator-based energy harvesting shoe and a dielectric electroactive polymer shoe. However, these systems were capable of generating only small amounts of electrical power: 250 mW and 800 mW, respectively.
Motion-based systems exploit muscle action as the origin of the mechanical work for human movement, and directly capture mechanical energy from joint motion during walking. For example, a knee-mounted energy harvester with a brushless DC rotary magnetic generator was proposed by Li, Q., et al., “Biomechanical energy harvesting: Apparatus and method,” in IEEE International conference on Robotics and Automation, 2008, pp. 3672-3677; and “Development of a biomechanical energy harvester,” Journal of Neuroengineering and Rehabilitation, 6:22, 2009. This system was mounted on a user's leg and harvested energy from the leg deceleration in each gait cycle with a control system turning on/off the power generation, based on an approach similar to regenerative braking used in hybrid cars. A pair of harvesters (one mounted on each of a user's legs) generated a total of 5 W of electrical power during walking at a speed of 1.5 m/s. In this mode, 1 W of metabolic power was required from the user to produce 1 W of electrical power. However, a drawback of this system is the mass added to the knee. Because the metabolic cost of carrying a given mass distally is considerably greater than that of carrying it proximally, walking while wearing such a system without power generation requires 20% more metabolic energy expenditure than walking without the device. Additionally, mounting the system to the side of the knee could potentially hinder normal movement and agility, and cause discomfort to the user.